DRAW(3)DRAW(3)NAMEdraw - screen graphics
bind -a #i /dev
ushort BGSHORT(uchar *p)
ulong BGLONG(uchar *p)
void BPSHORT(uchar *p, ushort v)
void BPLONG(uchar *p, ulong v)
The draw device serves a three-level file system providing an interface
to the graphics facilities of the system. Each client of the device
connects by opening /dev/draw/new and reading 12 strings, each 11 char‐
acters wide followed by a blank: the connection number (n), the image
id (q.v.) of the display image (always zero), the channel format of
the image, the min.x, min.y, max.x, and max.y of the display image, and
the min.x, min.y, max.x, and max.y of the clipping rectangle. The
channel format string is described in image(6), and the other fields
are decimal numbers.
The client can then open the directory /dev/draw/n/ to access the ctl,
data, colormap, and refresh files associated with the connection.
Via the ctl and draw files, the draw device provides access to images
and font caches in its private storage, as described in graphics(2).
Each image is identified by a 4-byte integer, its id.
Reading the ctl file yields 12 strings formatted as in /dev/draw/new,
but for the current image rather than the display image. The current
image may be set by writing a binary image id to the ctl file.
A process can write messages to data to allocate and free images,
fonts, and subfonts; read or write portions of the images; and draw
line segments and character strings in the images. All graphics
requests are clipped to their images. Some messages return a response
to be recovered by reading the data file.
The format of messages written to data is a single letter followed by
binary parameters; multibyte integers are transmitted with the low
order byte first. The BPSHORT and BPLONG macros place correctly for‐
matted two- and four-byte integers into a character buffer. BGSHORT
and BGLONG retrieve values from a character buffer. Points are two
four-byte numbers: x, y. Rectangles are four four-byte numbers: min x,
min y, max x, and max y. Images, screens, and fonts have 32-bit iden‐
tifiers. In the discussion of the protocol below, the distinction
between identifier and actual image, screen, or font is not made, so
that ``the object id'' should be interpreted as ``the object with iden‐
tifier id''. The definitions of constants used in the description
below can be found in draw.h.
The following requests are accepted by the data file. The numbers in
brackets give the length in bytes of the parameters.
A id imageid fillid public
Allocate a new Screen (see window(2)) with screen identifier id
using backing store image imageid, filling it initially with data
from image fillid. If the public byte is non-zero, the screen can
be accessed from other processes using the publicscreen interface.
b id screenid refresh chan repl r[4*4] clipr[4*4]
Allocate an image with a given id on the screen named by screenid.
The image will have rectangle r and clipping rectangle clipr. If
repl is non-zero, the image's replicate bit will be set (see
Refresh specifies the method to be used to draw the window when it
is uncovered. Refbackup causes the server to maintain a backing
store, Refnone does not refresh the image, and Refmesg causes a
message to be sent via the refresh file (q.v.).
The image format is described by chan, a binary version of the
channel format string. Specifically, the image format is the
catenation of up to four 8-bit numbers, each describing a particu‐
lar image channel. Each of these 8-bit numbers contains a channel
type in its high nibble and a bit count in its low nibble. The
channel type is one of CRed, CGreen, CBlue, CGrey, CAlpha, CMap,
and CIgnore. See image(6).
Color is the catenation of four 8-bit numbers specifying the red,
green, blue, and alpha channels of the color that the new image
should be initially filled with. The red channel is in the high‐
est 8 bits, and the alpha in the lowest. Note that color is
always in this format, independent of the image format.
c dstid repl clipr[4*4]
Change the replicate bit and clipping rectangle of the image
dstid. This overrides whatever settings were specified in the
d dstid srcid maskid dstr[4*4] srcp[2*4] maskp[2*4]
Use the draw operator to combine the rectangle dstr of image dstid
with a rectangle of image srcid, using a rectangle of image maskid
as an alpha mask to further control blending. The three rectan‐
gles are congruent and aligned such that the upper left corner
dstr in image dstid corresponds to the point srcp in image srcid
and the point maskp in image maskid. See draw(2).
If debugon is non-zero, enable debugging output. If zero, disable
it. The meaning of ``debugging output'' is implementation depen‐
e dstid srcid c[2*4] a b thick sp[2*4] alpha phi
Draw an ellipse in image dst centered on the point c with horizon‐
tal and vertical semiaxes a and b. The ellipse is drawn using the
image src, with the point sp in src aligned with c in dst. The
ellipse is drawn with thickness 1+2×thick.
If the high bit of alpha is set, only the arc of the ellipse from
degree angles alpha to phi is drawn. For the purposes of drawing
the arc, alpha is treated as a signed 31-bit number by ignoring
its high bit.
E dstid srcid center[2*4] a b thick sp[2*4] alpha
Draws an ellipse or arc as the e message, but rather than outlin‐
ing it, fills the corresponding sector using the image srcid. The
thick field is ignored, but must be non-negative.
Free the resources associated with the image id.
Free the the screen with the specified id. Windows on the screen
must be freed separately.
i id n ascent
Treat the image id as a font cache of n character cells, each with
l cacheid srcid index r[4*4] sp[2*4] left width
Load a character into the font cache associated with image cacheid
at cache position index. The character data is drawn in rectangle
r of the font cache image and is fetched from the congruent rec‐
tangle in image srcid with upper left corner sp. Width specifies
the width of the character—the spacing from this character to the
next—while left specifies the horizontal distance from the left
side of the character to the left side of the cache image. The
dimensions of the image of the character are defined by r.
L dstid p0[2*4] p1[2*4] end0 end1 thick srcid sp[2*4]
Draw a line of thickness 1+2×thick in image dstid from point p0 to
p1. The line is drawn using the image srcid, translated so that
point sp in srcid aligns with p0 in dstid. The end0 and end1
fields specify whether the corresponding line end should be a
square, a disc, or an arrow head. See line in draw(2) for more
N id in j name[j]
If in is non-zero, associate the image id with the string name.
If in is zero and name already corresponds to the image id, the
association is deleted.
n id j name[j]
Introduce the identifier id to correspond to the image named by
the string name.
o id r.min[2*4] scr[2*4]
Position the window id so that its upper left corner is at the
point scr on its screen. Simultaneously change its internal (log‐
ical) coordinate system so that the point log corresponds to the
upper left corner of the window.
Set the compositing operator to op for the next draw operation.
(The default is SoverD).
p dstid n end0 end1 thick srcid sp[2*4] dp[2*2*(n+1)]
Draw a polygon of thickness 1+2×thick. It is conceptually equiva‐
lent to a series of n line-drawing messages (see L above) joining
adjacent points in the list of points dp. The source image srcid
is translated so that the point sp in srcid aligns with the first
point in the list dp. The polygon need not be closed: end0 and
end1 specify the line endings for the first and last point on the
polygon. All interior lines have rounded ends to make smooth
P dstid n wind ignore[2*4] srcid sp[2*4] dp[2*2*(n+1)]
Draw a polygon as the p message, but fill it rather than outlining
it. The winding rule parameter wind resolves ambiguities about
what to fill if the polygon is self-intersecting. If wind is ~0,
a pixel is inside the polygon if the polygon's winding number
about the point is non-zero. If wind is 1, a pixel is inside if
the winding number is odd. Complementary values (0 or ~1) cause
outside pixels to be filled. The meaning of other values is unde‐
fined. The polygon is closed with a line if necessary.
r id r[4*4]
Cause the next read of the data file to return the image pixel
data corresponding to the rectangle r in image id.
s dstid srcid fontid p[2*4] clipr[4*4] sp[2*4] n
Draw in the image dstid the text string specified by the n cache
indices into font fontid, starting with the upper left corner at
point p in image dstid. The image drawn is taken from image
srcid, translated to align sp in srcid with dp in dstid. All
drawing is confined to the clipping rectangle clipr in dstid.
x dstid srcid fontid dp[2*4] clipr[4*4] sp[2*4] n bgid
Like the string drawing s command, but fill the background of each
character with pixels from image bgid. The image bgid is trans‐
lated so that the point bp aligns with the point dp in dstid.
S id chan Attach to the public screen with the specified id. It
is an error if the screen does not exist, is not public, or does
not have the channel descriptor chan for its associated image.
t top n n*id
Send n windows to the top (if t is non-zero) or bottom (if t is
zero) of the window stack. The window is specified by the list of
n image ids are moved as a group, maintaining their own order
within the stack.
Flush changes from a soft screen, if any, to the display buffer.
y id r[4*4] buf[x*1]
Y id r[4*4] buf[x*1]
Replace the rectangle r of pixels in image id with the pixel data
in buf. The pixel data must be in the format dictated by id's
image channel descriptor (see image(6)). The y message uses
uncompressed data, while the Y message uses compressed data. In
either case, it is an error to include more data than necessary.
Reading the colormap returns the system color map used on 8-bit dis‐
plays. Each color map entry consists of a single line containing four
space-separated decimal strings. The first is an index into the map,
and the remaining three are the red, green, and blue values associated
with that index. The color map can be changed by writing entries in
the above format to the colormap file. Note that changing the system
color map does not change the color map used for calculations involving
m8 images, which is immutable.
The refresh file is read-only. As windows owned by the client are
uncovered, if they cannot be refreshed by the server (such as when they
have refresh functions associated with them), a message is made avail‐
able on the refresh file reporting what needs to be repainted by the
client. The message has five decimal integers formatted as in the ctl
message: the image id of the window and the coordinates of the rectan‐
gle that should be refreshed.
Most messages to draw can return errors; these can be detected by a
system call error on the write(see read(2)) of the data containing the
erroneous message. The most common error is a failure to allocate
because of insufficient free resources. Most other errors occur only
when the protocol is mishandled by the application. Errstr(2) will
The Refmesg refresh method is not fully implemented.
The colormap files only reference the system color map, and as such
should be called /dev/colormap rather than /dev/draw/n/colormap.